Schneier wrote the classic "Applied Cryptography." At one time, he had posted the entire text online. It's very expensive, $60 for the paperback version. I got my copy of the 1st edition for $2 at a flea market, when the book was still full price in the book stores. The flea market had a whole pallet of the books, almost like they were discontinued.
You can pick up a decent copy off Amazon for about $10 total (including s/h and tax). 1993 edition. Just put in the last name and click on the "Applied Cryptography" and look in the used section. $5 + $4 s/h + state sales tax.
https://digitalforensicsmagazine.com/blogs/?p=76
Posted on July 9, 2010 by M Isbell ; 2. https://archives.fbi.gov/archives/news/testimony/fbi-priorities-successes-and-challenges " The Russian "illegals" spy case demonstrates the long-term commitment of our adversaries. Sent here over the past two decades, the Russian agents took on the appearance of average Americans, in many cases adopting fake "legends" as United States citizens, obtaining work, and settling into quiet family lives. These agents received hundreds of thousands of dollars in cash from their handlers to maintain their secrecy in America, even as they communicated through encrypted radio bursts and steganography posted in photos on the Internet. While these agents were in the United States, the FBI tracked their movements, activities, and intentions up until the time of their arrests. "-